Numerous hydrocarbon conversion processes are widely used to alter the structure or properties of hydrocarbon streams. For example, isomerization processes rearrange the molecular structure from straight chain paraffinic hydrocarbons to more highly branched hydrocarbons that generally have a higher octane rating or increased utility as substrates for other conversion processes. Alkylation processes alkylate hydrocarbon alkylation substrates, such as aromatics or paraffins, with hydrocarbon alkylating agents, such as olefins, to produce motor fuels and useful industrial chemicals such as ethylbenzene, cumene, and linear alkyl benzenes that are used to make detergents. Additional processes include dehydrogenation, transalkylation, reforming, and others. Operating conditions and methods for carrying out these processes are well known by those skilled in the art.
Many of these processes share the common feature of using a catalyst in the presence of one or more materials that enhance the effectiveness of the catalyst in the reaction zone. These performance enhancing materials can operate in many ways, such as increasing or attenuating catalyst activity, neutralizing catalyst poisons, or solubilizing catalyst or feed contaminants. Such performance enhancement materials may be chemically or physically sorbed on the catalyst or dispersed in the hydrocarbon stream.
Where the hydrocarbon product stream leaving a hydrocarbon conversion zone contains the performance enhancing material or beneficent material, methods are sought for preventing contamination of the hydrocarbon product with the beneficent material and the loss of this beneficent material to the product stream. For example, many isomerization processes employ a highly effective platinum on chlorided alumina catalyst system in the reaction zone. The chlorided catalyst requires a continual addition of chloride to replace chloride lost from the surface of the catalyst in to the product stream. Hydrogen chloride and/or volatile organic chlorides escape from the process via a stabilizer overhead stream and, apart form the loss of chloride, pose environmental concern. In addition to the loss of chlorides and environmental concerns, chloride loss hinders the operation of chloride promoted isomerization zones in other ways. For example, the recycle of hydrogen or hydrocarbons through a zeolitic adsorption bed or a zeolitic conversion zone is not practical when a chloride type catalyst is used in the isomerization reaction zone, unless hydrogen chloride is removed from the recycle stream. Hydrogen chloride that is produced by the addition of chloride to the isomerization zone or that is released from the isomerization catalyst results in significant amounts of hydrogen chloride leaving in the effluent from the isomerization zone. Contact of this hydrogen chloride with a zeolite in, say, an adsorption bed or in a catalytic conversion zone, particularly in the presence of moisture and high temperature, will decompose the matrix structure of many zeolites, thereby destroying any adsorptive or catalytic function. Therefore, absent chloride neutralization methods, chlorided catalyst systems generally have insufficient compatibility with many zeolitic adsorbents or catalysts to permit simultaneous use.
Alkylation of hydrocarbons presents another case where contamination by a performance enhancement material can pose concern. Alkylation processes can use a solid chlorided alumina catalyst in the alkylation reactor, with the chloride acting as a performance enhancement material for the catalyst. In the course of alkylation, some chloride is lost from the catalyst into the product stream. In addition, the current commercial versions of these solid alkylation catalysts tend to experience fairly rapidly deactivation, which necessitates frequent regeneration, which in turn usually leads to the loss of more chloride from the catalyst into one or more regeneration effluent streams. Unless some or most of the lost chloride in the product stream and the regeneration effluent stream(s) is recovered and returned to the catalyst, the costs of neutralizing chlorides and of supplying fresh or make-up chloride to the catalyst can render alkylation processes that use solid chlorided catalysts less economical. Thus, methods have been sought for recovering and recycling materials, such as hydrogen chloride, which act to enhance or benefit the performance of catalysts in conversion zones when such materials are carried from the conversion zone by a hydrocarbon-containing effluent stream.
Since the 1960s, the chemical and petrochemical industries are increasingly using membranes in a broad variety of separation and recovery applications. A membrane is a thin barrier having two sides and separating two fluids. In a membrane process, a feed stream passes to one side of the membrane, which is commonly called the feed side or nonpermeate side of the membrane. The feed stream contains a permeable component and a nonpermeable component, which is used herein to refer to a component that has a permeance that is less than that of the permeable component. The permeable component selectively passes through the membrane and is recovered on the reverse side, or permeate side, of the membrane in a stream which is called the permeate. The portion of the feed stream that does not selectively pass through the membrane, including the nonpermeable component, is recovered from the nonpermeate side of the membrane in a stream which is called the nonpermeate and which is also commonly referred to as the concentrate, the retentate, or the residue. General information on membrane separation processes can be found in Perry's Chemical Engineers' Handbook, Sixth Edition, edited by R. H. Perry and D. W. Green, published by McGraw-Hill Book Company, New York, in 1984.
In order for the permeable component to permeate from the nonpermeate side to the permeate side of the membrane at a particular location of a membrane, the local partial pressure of the permeable component at that particular location at or near the surface of the membrane must be greater on the nonpermeate side of the membrane than on the permeate side of the membrane. Local partial pressure of the permeable component means the product of the mole fraction of the permeable component and the total pressure, both determined locally at a given particular point at or near the surface of the membrane. In practice, the local mole fraction and local total pressure cannot be measured precisely at the surface of the membrane. Rather, the local mole fraction and total pressure are determined at least in part by the bulk flow rates, compositions, and pressures of the stream that are contacted with the membrane, and by controlling these streams, suitable local partial pressures of the permeable component can be controlled on both sides of the membrane.
The separation of gas streams using dense membranes that are nonporous and yet permeable is well known. Dense membranes consist of a dense film through which a pressure and concentration gradient will force the diffusion of certain components. The relative rates of transport of various components through the dense film does not necessarily depend on the size of the components, as much as it depends on the diffusivity and solubility of the components. See the article entitled "Membrane Technology," written by Richard W. Baker, appearing at pp. 135-193 in Vol. 16 of the Kirk-Othmer Encyclopedia of Chemical Technology, 4.sup.th Edition, published by John Wiley, in New York, in 1995.
Two of the chief criteria for a dense membrane separation process are permeance and selectivity. At this point, it is useful for the sake of clarity to define these terms:
i. Permeance
In simple permeation, the permeation flux of a species i across a membrane may be expressed by the formula, EQU R.sub.i =P.sub.i *(y.sub.H *H-y.sub.L *L)
where R.sub.i equals the rate of permeation in units of standard volume of species i per unit of time per unit of membrane cross-sectional area, P.sub.i equals the permeance, y.sub.H equals the mole fraction of species i on the high-pressure or nonpermeate side of the membrane, H equals the pressure on the high-pressure side of the membrane, y.sub.L equals the mole fraction of species i on the low-pressure or permeate side of the membrane, and L equals the pressure on the low-pressure side of the membrane. Thus, permeance has units of standard volume of species i per unit of time per unit of membrane cross-sectional area per unit of pressure.
ii. Selectivity
Selectivity is defined as the ratio of the permeance of species i relative to the permeance of another species j and can be expressed by the equation, EQU S.sub.ij =P.sub.i /P.sub.j
In this equation, the convention is to put the numerically larger permeance in the numerator and the numerically smaller permeance in the denominator, so that the higher the selectivity of a membrane, the more desirable is the use of that membrane in a process for separating species i and j.
Examples of dense membranes include sulfonated fluorinated polysulfone membranes that comprise polymers, including copolymers, which have the polymer repeat unit of the following general type of structure in the polymer or copolymer, which is hereinafter referred to as formula (I): ##STR2##
In the above formula (I), S is the sulfonic acid group (SO.sub.2 OH) or its satisfied form, and n represents the average number of polymer repeat units in the polymer or copolymer. Hackh's Chemical Dictionary, Third Edition, edited by Julius Grant, published by The Blakiston Company, Inc., New York, in 1953, defines the term "salify" as "to form a salt." The term "satisfied form of the sulfonic acid group" as used herein means a form of the sulfonic acid group wherein the hydrogen atom of the hydroxyl group of the sulfonic acid group is replaced with a cation or cationic group. The salified form typically contains an ammonium group, an alkali metal atom, an alkaline earth metal atom, a transition metal atom, or an organic cation group. The polymer or copolymer has a molecular weight of generally above about 10,000 and preferably from about 25,000 to about 80,000. The polymer or copolymer has a degree of substitution (DS) of S groups of from about 0.2 to about 4. These sulfonated fluorinated polysulfone membranes are disclosed in U.S. Pat. No. 4,971,695 (Kawakami et al.), which teaches their use in separating gas mixtures such as air, as well as mixtures comprising hydrogen/nitrogen, hydrogen/methane, oxygen/nitrogen, ammonia/nitrogen, carbon dioxide/oxygen, carbon dioxide/methane, and hydrogen sulfide/methane. It is also known that these sulfonated fluorinated polysulfone membranes are used to separate gas mixtures of water/air and water/methane, with water permeating through the membrane.
Although sulfonated fluorinated polysulfone membranes have been used for separating streams, these membranes have not been used for separating and recovering hydrogen chloride from streams. Therefore, a method is sought that uses membranes for recovering and recycling hydrogen chloride from hydrocarbon-containing streams.